At the high speeds at which sheet-fed offset presses are typically run, the sheets to be printed are removed from a feed stack and conveyed in a part-overlapping relationship to a set of lay marks (positioning guides) by way of a conveying table, such as a suction belt table described in U.S. Pat. No. 4,647,033 (corresponding to German Pat. DE 3 138 481 C2). The front edge of the sheet is aligned on a front lay mark in the direction of sheet movement, such as by the additional conveying means of U.S. Pat. No. 4,651,984 (corresponding to German Pat. DE 3 331 662 C2), and the side edge of the sheet is aligned on a side lay. When this aligning step is complete, a gripper, usually cam-operated, engages the sheet. Ordinarily, the lay marks are then pivoted away and the gripper transfers the sheet directly to a separate gripper system of an impression cylinder.
The most widely used sheet-fed offset presses have a blanket cylinder eccentrically mounted at both ends so that the blanket cylinder can be completely thrown off the impression cylinder and a plate cylinder by rotation of this eccentric mounting. This mode of operation, when all cylinders have been thrown off, is known as the "printing off" state of operation. For inking, the blanket cylinder is first thrown onto the plate cylinder but is not thrown onto the impression cylinder, and then for printing, the blanket cylinder is also thrown onto the impression cylinder so that all three cylinders operate together. When all three cylinders operate together, the press is in the "printing on" state of operation. The eccentric bearings that enable these throw-on and throw-off movements are operated by cam followers, cam follower levers and actuatable pawls. For proper operation, the throw-on and throw-off of the blanket cylinder with respect to the plate cylinder and impression cylinder must occur at the instant when the gaps of the corresponding printing unit cylinders register with one another. Similarly, as is known from the prior art, the pivoting-away movements of the lay marks and the engagement of the sheet and transfer movements of the gripper are produced by a transmission system also having cam followers, cam follower levers, and pawls which can be engaged to interrupt these movements.
It is also known from the prior art to position sheet sensors near the lay marks for detecting whether the sheet is properly aligned on the lay marks in sufficient time for gripping (detection of late sheets) and to verify that the sheet is engageable by the gripper over its entire width (detection of skewed sheets). Sheet sensors of this kind usually are based on optoelectronic sensing, such as disclosed in German Pat. DE 3 907 583 A1. If the sensing result is negative--i.e., the sheet is not aligning on the lays correctly--the auxiliary gripper does not engage the sheet, the lay marks do not pivot away and the blanket cylinder is thrown off the impression cylinder and plate cylinder. In other words, the press is put into the printing off mode.
These sheet sensors monitor sheet entry both in the printing on and printing off states of operation. In the printing off state, when a sheet is properly sensed, the sheet is gripped, transferred to the impression cylinder, and the blanket cylinder is thrown first onto the plate cylinder and then onto the impression cylinder carrying this first sheet. When this has occurred, the press has changed to the printing on state. In the printing on state subsequent sheets are gripped and transferred to the impression cylinder, but throwing-on movements are unnecessary since the cylinders have already been thrown-on.
When the press is running at high press speeds, the sheets tend to rebound from the lay marks before settling properly and aligning along the lay marks correctly. If the sensor attempts to detect a sheet before it has properly settled (shortly after arrival), sheet movement will be halted.
It is known from British Patent 2,071,064 (corresponding to German Pat. DE 3 044 643 A1) that triggering the mechanical elements for release or stoppage of the sheet, or actuation, is possible even at the maximum press speed. However, because these elements take time to operate, the decision on which elements to actuate must be made at a time that depends on the speed of the press. Therefore, the time at which the sheet must be detected is determined by an angle of rotation from a single-revolution shaft of a cylinder. For example, this time or angle can be derived from a timer or angular position sensor on the shaft. This "instant of sheet sensing" must be chosen so that in the printing on state there is still sufficient time to stop an improper sheet entry, and so that in the printing off state there is still sufficient time for the cylinder throw-on movements to occur and also to load a sheet for printing.
A disadvantage of a rigid association between the instant of sheet sensing and a fixed angle of rotation of a single-revolution shaft is that in some printing units and gripper arrangements, the instant of sheet sensing must occur at an early enough angle so that it is still possible to trigger the start of printing and the release of the sheet for movement. A distinctive feature of these printing units and gripper arrangements is that the angle (time) of a single-revolution shaft at which the blanket cylinder is required to be thrown onto the plate cylinder (so that the gaps of these cylinders correspond) occurs before the angle (time) at which release of the sheet for movement is possible, (pivoting away of the lay marks, triggering of the gripper) and all movements must begin before the angle (time) at which the blanket cylinder can be thrown onto the impression cylinder.
With such a system, the instant of sheet sensing must be chosen so that it will occur before any of the movements begin. The first movement that could possibly occur is the throwing-on of the plate cylinder to the blanket cylinder, when printing is off. Therefore, the instant of sheet sensing must take place prior to this first throw-on movement. In other words, the latest possible angle that can be chosen must precede the angle at which this first throw-on movement must begin.
In the printing off state, there is always sufficient time for settling to occur, and the angle chosen corresponding to the instant of sheet sensing presents no problems. However, when the press is in the printing on state, problems occur at high printing speeds because the settling time of a sheet often exceeds the time it takes for this chosen sensing angle to be reached by the rotating shaft. Consequently, sheet movement is frequently stopped and the press goes into the printing off state, even though the sheet would have settled in plenty of time to be properly gripped. This phenomenon occurs almost every time the sheets are supplied to the press in an overlapping relationship at high press speeds.
German patent DE 4 019 293 A1 describes a facility for monitoring sheets on the lay marks of presses wherein the instant of sensing is variable and is selected in inverse proportion to press speed. The alleged aim is to increase the time available for the sheet to settle on the lay marks. However, a facility of this kind only makes allowances for the time taken between an actuation signal given and the actual operation of the mechanical elements, by advancing the actuation signal at high press speeds. Accordingly, the instant of sheet sensing must also be advanced. Sheet settlement time is therefore not increased at all.